A comprehensive physical model will be developed for the effects of foreign ion substitution and laser irradiation on the dissolution kinetics of hydroxyapatite. Foreign ions to be studied are carbonate, fluoride, strontium and magnesium. The major hypothesis to be tested is that when these ions are incorporated into the apatite lattice, the free energy of the crystal (and consequently its solubility) is altered, even though the dissolution rate controlling phase may not change. Studies done with carbonate apatite samples suggest that there may be a linear free energy relationship between carbonate content and this crystal free energy. Specific hypotheses for individual ions are as follows. Carbonate is proposed to alter solubility but not participate in the rate controlling surface phase; fluoride, on the other hand, is thought to readily participate. The mechanism(s) by which strontium and magnesium act may be concentration dependent. Laser irradiation reduces solubility of hydroxyapatite, apparently by annealing away the more soluble crystalline dissolution sites. The interaction of foreign ion substitution on this effect will be characterized. The studies to be done during this project will collectively provide a broadly based mechanistic interpretation for foreign ion and laser effects. These proposed mechanisms will be directly relatable to crystal structure and energetics and therefore these studies will constitute a significant step toward an eventual quantitative atomic level understanding of foreign ion and laser effects.